Fix: Merge individual commit_strategy fields from defaults

Previously, when a workflow had ANY commit_strategy fields, it would
completely ignore all defaults. This meant workflows with partial
commit_strategy (e.g., only pr_title and pr_body) wouldn't inherit
commit_message or use_pr_template from defaults.

Changes:
- Modified YAMLConfig.SetDefaults() to merge individual fields
- Modified WorkflowConfig.SetDefaults() to merge individual fields
- Fields now inherited: Type, CommitMessage, PRTitle, PRBody, UsePRTemplate
- AutoMerge intentionally NOT inherited for safety when workflow has
  its own commit_strategy

Tests:
- Added comprehensive tests in types/config_test.go
- Updated existing test in services/main_config_loader_test.go
- All tests pass

Author: cbullinger